Alex Salmond has said there would be an "inevitable investigation" over allegations that the Treasury leaked "market-sensitive" information about Royal Bank of Scotland's proposal to register itself in England if Scotland votes for independence.

The First Minister and SNP leader said the alleged BBC briefing by a "Treasury source" was "a matter of extraordinary gravity", and urged the BBC to co-operate with the probe that must follow.

Speaking at a press conference for international journalists in Edinburgh, Mr Salmond said: "A Treasury source told the BBC that it had discussed the plans with RBS.

"The Treasury, officials or ministers, are not allowed to brief market-sensitive information.

"Market-sensitive information, and it's a basic rule, cannot be released prior to the market announcement at 7am this morning.

"RBS share price changed overnight. This is a matter of extraordinary gravity.

"I've always respected, and I will continue to respect, the journalistic right to maintain and protect sources.

"But I know that the BBC will want to co-operate with the inevitable investigation by the Cabinet Secretary to the briefing of this information, given that the briefing of information - even if we weren't in a referendum campaign, even if there weren't purdah rules which are meant to apply to government - the briefing of market information is as serious a matter as you can possibly get."

Some members of the audience - which was packed with supporters as well as international journalists - laughed and applauded when the First Minister spoke of the BBC's "impartial role as public sector broadcaster".

BBC political editor Nick Robinson was also heckled from the audience while questioning the First Minister.

RBS, which has been based in Scotland since 1727, said it would be necessary to re-domicile the bank's holding company and its main operating entity to England if Scotland votes Yes.

It said the decision to re-domicile should have no impact on everyday banking services and it would retain a significant level of its operations and employment in Scotland.

Lloyds Banking Group issued similar advice this morning, while Standard Life has also advised investors it is "planning for new regulated companies in England to which we could transfer parts of our business if there was a need to do so".

RBS said there are a number of "material uncertainties" arising from the Scottish referendum vote which could have a bearing on the bank's credit ratings, and the fiscal, monetary, legal and regulatory landscape.

It added: "For this reason, RBS has undertaken contingency planning for the possible business implications of a Yes vote. RBS believes that this is the responsible and prudent thing to do and something that its customers, staff and shareholders would expect it to do."

A Lloyds spokesman said the group - which also includes Scottish Widows - had been contacted by concerned customers, staff and stakeholders about its plans in the event of a Yes vote.

The spokesman said: "This is a legal procedure and there would be no immediate changes or issues which could affect our business or our customers.

"As a group we are committed to supporting our customers across Scotland and the rest of the UK."

Treasury Financial Secretary David Gauke said: "Looks like an independent Scotland will have more pandas than banks or insurance companies."

Clydesdale Bank, which is part of National Australia Bank, also said its contingency plans for a Yes vote included re-registering as an English company to mitigate risks and provide increased certainty for customers.

Edinburgh-based TSB, which recently returned to the London stock market after two decades, said it will establish additional legal entities in England.

Mr Salmond added that the No campaign has been "caught red-handed as being part of a campaign of scaremongering".

"I'm not making that position against any of the companies concerned," he said.

"But quite clearly if you brief market sensitive information last night to one broadcaster which was meant to be released at 7am this morning, it put this Treasury fingerprints all over this story, and it provides a spectacular example of the sort of campaign tactics of intimidation and bullying that have served the No campaign so badly."

Responding to the banks' announcements, former RBS chairman and chief executive Sir George Mathewson said: "There is no real jobs, revenues or investment impact in any of these technical announcements.

"They are also very similar to what has been said before. In any event, of course there will be a currency union in the interests of both Scotland and the rest of the UK.

"The UK Government-driven scaremongering and its political gamesmanship is of detriment to both market confidence and the quality of democratic debate.

"Voters will not be swayed by these tactics. The momentum remains with the Yes campaign towards a more prosperous, stronger and fairer Scotland."

A BBC spokeswoman said: "We note that Alex Salmond has called for an inquiry by the Cabinet Secretary and we will await a decision on that request.

"The story in question was accurate and BBC News exercised normal editorial judgments."